.NET Framework 3.5 is so old it’s not available by default on Windows (maybe not available at all on the latest Windows), you’d probably have to work with ancient developer tooling to work with it, it’s probably unsupported and has security issues.

And that’s ignoring how you’re essentially severely handicapping yourself in terms of what is possible.

Unless you’re in an environment stuck 20 years in the past (which implies serious security liabilities considering they must be a Microsoft shop), this is a mind bogglingly bizarre strategy.