Are old rev*.dat file safely deletable?
No. You need the rev*.dat files as much as you need the blk*.dat files. If Bitcoin Core cannot find the rev*.dat, it will need to regenerate them by reindexing the databases. Removing them effectively corrupts the databases.

If you want to save disk space, you can use pruning instead. This will have Bitcoin Core properly remove blk*.dat and rev*.dat files to save disk space.