I believe they're under https://dumps.wikimedia.org/be_x_oldwiki/20230501/ which is the DB name for it (until/unless the DB renaming task gets done).

Thank you. Found it now. The names on https://dumps.wikimedia.org/backup-index.html are presented not in alphabetical order and this got me confused.
Also before opening this phabricator ticket, I checked some mirrors and found "be_x_oldwiki" there. But the https://mirror.accum.se/mirror/wikimedia.org/dumps/be_x_oldwiki/ and https://wikidata.aerotechnet.com/be_x_oldwiki/ directories are empty and this got me worried.

Thanks again. I guess, this issue is resolved.

In T336156#8832876, @Ssvb wrote:

Thank you. Found it now. The names on https://dumps.wikimedia.org/backup-index.html are presented not in alphabetical order and this got me confused.
Also before opening this phabricator ticket, I checked some mirrors and found "be_x_oldwiki" there. But the https://mirror.accum.se/mirror/wikimedia.org/dumps/be_x_oldwiki/ and https://wikidata.aerotechnet.com/be_x_oldwiki/ directories are empty and this got me worried.

Thanks again. I guess, this issue is resolved.

Yeah, this is T111876: Figure out how renaming languages without database names is supposed to work with SiteMatrix, and implement for be-x-old -> be-tarask. I guess the task should be switched to "make it easier to find dumps under unusual names"?