I do something similar with build trees, naming them +build, +cross-arm etc.

This convention was suggested by the GNU Arch version control system years ago (maybe 20??), but it's really useful for the same tab completion reason and I have kept it for almost two decades, even when I switched to git.

It was suggested by Tom Lord (RIP), who used it heavily long before he wrote GNU Arch.

File names or directories starting with a comma where considered “junk”, and ones with a plus sign I think where considered “precious”.