The aufs package last saw a maintainer upload in September 2019 and was last-updated (by a NMU) in October 2019. It has had broken build-dependencies in testing for half a year now (since Linux 5.3.9-3 migrated to testing in November 2019).
According to dak rm the aufs source-package has two reverse-dependencies, aufs-tools and fsprotect neither of which has any reverse-dependencies. Adrian filed a rc bug in November 2019 which received no maintainer response, however the package was not autoremoved from testing due to aufs and aufs-tools being considered a "key packages" due to high popcon. This popcon actually seems to be growing in both absolute and percentage terms. I presume the high popcon is due to some deriviative (hence debian-derivatives and debian-live in cc) using aufs in their live image builds (as far as I can tell debian's own live images seem to use overlayfs instead nowadays). aufs does seem to still be maintained upstream with upstream claiming support for Linux 5.6. According to contributors.debian.net Jan Luca Naumann (the aufs maintainer) was last active in September 2019. Jan: are you still around? and if so do you still intend to maintain the aufs package? if not is someone else going to step up to the plate? or should these packages be removed from testing?
