I haven’t seen any breakage, although you may find documentation assumes SystemD. Debian maintains init freedom, and support for sysVinit was improved in Bullseye, so it’s not being forgotten about.

If you don’t fancy going that route, there are Debian forks that are designed to be SystemD free such as Devuan or MX linux, which defaults to sysVinit. I’ve not tried either, but they seem well regarded, and I’m sure there are others too.