The major versions of all the libraries were bumped on 2021-04-27. Follows a summary of backwards-incompatible changes.

libavcodec

• old audio/video decoding APIs avcodec_decode_video2 and avcodec_decode_audio4 were removed; they are replaced by avcodec_send_packet/avcodec_receive_frame, which decouple input and output and can return any number of output frames for a single input packet
• old audio/video encoding APIs avcodec_encode_video2 and avcodec_encode_audio2 were removed; they are replaced by avcodec_send_frame/avcodec_receive_packet, which decouple input and output and can return any number of output packets for a single input frame
• AVCodecContext.coded_frame removed without replacement
• AVCodecContext.side_data_only_packets removed, codecs (currently only flacenc) now always behave as if this field was set to 1
• AVCodecContext.vbv_delay removed, replaced by AV_PKT_DATA_CPB_PROPERTIES side data
• AVCodecContext.rtp_callback removed without replacement
• AVCodecContext.*_bits removed without replacement
• following AVCodecContext fields removed, replaced by encoder-private options
  • coder_type and FF_CODER_TYPE*
  • b_frame_strategy
  • mpeg_quant
  • prediction_method and FF_PRED_*
  • pre_me
  • scenechange_threshold
  • noise_reduction
  • me_penalty_compensation
  • brd_scale
  • chromaoffset
  • b_sensitivity
  • context_model
  • frame_skip_*
  • min/max_prediction_order
  • timecode_frame_start
  • rtp_payload_size
• AVPacket.convergence_duration removed, use AVPacket.duration instead
• AVPacket API for pre-refcounted memory management were removed
  • av_dup_packet
  • av_copy_packet
  • av_copy_packet_side_data
  • av_free_packet
  users should use refcounted packet API instead
• av_packet_merge_side_data and av_packet_split_side_data removed without replacement, packets with merged side data are no longer supported
• AVPicture and its related APIs removed; it is replaced either by AVFrame API or imgutils in libavutil
• old bistream filtering API (using AVBitStreamFilterContext) removed, replaced by the API in libavcodec/bsf.h
• avcodec_copy_context removed, it never makes sense for users to call this function
• avcodec_get_context_defaults3 removed, users should allocate a new codec context instead of resettting an old one
• av_get_codec_tag_string replaced by av_fourcc_make_string/av_fourcc2str
• avcodec_get_chroma_sub_sample replaced by av_pix_fmt_get_chroma_sub_sample
• AVCodecContext accessors removed, AVCodecContext fields should be accessed directly
• AVHWAccel and its related functions removed from public API, as there was no reason for them to be user-visible
• av_lockmgr_register and AVLockOp removed, libavcodec handles locking internally
• codec registration APIs removed, all codecs are always registered
  • avcodec_register
  • avcodec_register_all
• av_codec_next replaced by av_codec_iterate
• AVCPBProperties.max/min/avg_bitrate types changed from int to int64_t
• avcodec_get_pix_fmt_loss replaced by av_get_pix_fmt_loss
• avcodec_find_best_pix_fmt_of_2 replaced by av_find_best_pix_fmt_of_2
• avcodec_find_best_pix_fmt2 removed
• av_parser_change removed; dump_extradata, remove_extra or extract_extradata should be used instead
• FF_SUB_TEXT_FMT_ASS_WITH_TIMINGS removed, ASS subtitles are always decoded without timings
• mpegvideo encoders' rc_strategy option removed, as it had no effect
• av_vdpau_get_profile removed, users should call av_vdpau_get_profile to set up VDPAU decoding
• libopenh264 encoder's 'slice_mode' option removed, as it is redundant with 'slices'/'max_nal_size' options
• libopenh264 encoder's 'cabac' option replaced with 'coder cabac'
• 'nvenc' encoder renamed to 'h264_nvenc'
• struct vaapi_context removed, AVCodecContext.hw_frames_ctx in combination with the API in libavutil/hwcontext.h should be used instead

libavformat

• AVFormatContext.open_cb replaced by io_open/io_close
• AVStream.codec removed; stream information is now exported through AVStream.codecpar; for decoding/encoding the users should allocate a separate codec context
• HTTP and RTSP protocol's 'user-agent' option renamed to 'user_agent'
• HLS muxer's 'hls_wrap' option removed, users should use 'hls_list_size'/'hls_flags delete_segments' instead
• HLS muxer's 'user_localtime'/'use_localtime_mkdir' options removed, users should use 'strftime'/'strftime_mkdir' instead
• AVFMT_FLAG_KEEP_SIDE_DATA removed, it has had no effect since last major bump
• mov muxer/demuxer no longer read/export the "rotate" tag, use AV_PKT_DATA_DISPLAYMATRIX instead
• AVFormatContext accessors removed, AVFormatContext fields should be accessed directly
• custom AVIOContext instances may no longer return 0 on EOF, they should return AVERROR_EOF or another appropriate error code
• APIs related to ffserver removed without replacement, as ffserver itself has been removed
  • AVStream.recommended_encoder_configuration
  • av_stream_get_recommended_encoder_configuration
  • av_stream_set_recommended_encoder_configuration
• AVFormatContext.filename replaces with AVFormatContext.url
• format registration APIs removed, all formats are always registered
  • av_register_all
  • av_register_input_format
  • av_register_output_format
• av_iformat_next replaced with av_demuxer_iterate
• av_oformat_next replaced with av_muxer_iterate
• DASH muxer's 'min_seg_duration' option replaced with 'seg_duration'
• AVFMT_FLAG_MP4A_LATM removed, as it had no effect since 2011
• all instances of AVOutputFormat/AVInputFormat visible through the API are now const-qualified this is only a type change, as the caller was never allowed to modify them
• av_demuxer_open removed, use avformat_open_input with an options dictionary
• AVChapter.id type changed from int to int64_t

libavutil

• old VAAPI pixel formats
  • AV_PIX_FMT_VAAPI_IDCT and AV_PIX_FMT_VAAPI_MOCO were never-used - just removed
  • AV_PIX_FMT_VAAPI_VLD renamed to just AV_PIX_FMT_VAAPI
• QP tables in AVFrame were removed, as they were not flexible enough for new codecs
  • qscale_table
  • qstride
  • qscale_type
  • qp_table_buf
  they were replaced by the API in libavutil/video_enc_params.h
• AVFrame.error removed without replacement
• AVFrame.pkt_pts replaced with AVFrame.pts
• AVFrame accessors were removed, AVFrame fields should be accessed directly
• types of arguments/return values of the following functions changed from (unsigned) int to size_t:
  • av_adler32_update
  • av_hash_update
  • av_md5_update
  • av_md5_sum
  • av_murmur3_update
  • av_ripemd_update
  • av_sha_update
  • av_sha512_update
  • av_buffer_alloc
  • av_buffer_allocz
  • av_buffer_create
  • av_buffer_realloc
  • av_buffer_pool_init
  • av_buffer_pool_init2
• AVBufferRef.size changed from int to size_t
• "compressed" AVComponentDescriptor fields
  • step_minus1
  • depth_minus1
  • offset_plus1
  were replaced by same fields without the offset
• AV_PIX_FMT_FLAG_PSEUDOPAL removed, pixel formats previously flagged with it are no longer treated as paletted