Why D-Link DIR-869 rev. A1 is not added to the snapshot? I would like to build own firmware with snapshot but how to do it if can't download openwrt-imagebuilder-ar71xx-generic for the D-Link DIR-869 rev. A1?
DIR-869 A1 is ar71xx target. This target has been dropped in August 2020 and is no longer available in master (snapshots).
If you want to build your own image, use 19.07.5, where the ar71xx target is still present.
Also, you are welcome to work on adding the device to the new target ath79 which replaces ar71xx and uses DTS which is much easier to learn than the C language