The check_block_header function (which uses the getblockheader RPC method) is already compatible with pruned blockchains. It's only the getblock RPC that fails on pruned blockchains. The commit you made to change the initialization routine will allow people to use the wrong daemon at startup (e.g. Bitcoin ABC on the regular BTC network) and is dangerous.

For reference, I added support for pruned mode in this commit, with the fork protection and some renaming done here.