Maintaining the blockfilterindexes in prune mode is possible and may lead to efficient p2p based rescans of wallets (restore backups, import/sweep keys) beyond the prune height (rescans not part of that PR).
This PR allows running the blockfilterindex(es) in conjunction with pruning.

Bitcoind/Qt will shutdown during startup when missing block data has been detected ([re]enable -blockfilterindex when we already have pruned)
manual block pruning is disabled during blockfilterindex sync
auto-pruning is delayed during blockfilterindex sync

ToDos:

Functional tests

Article posted 2 weeks ago