Are chains of 26 unconfirmed transactions prohibited by the wallet in Bitcoin Core 31.0?

You can have a chain longer than 25 unconfirmed transactions in the mempool, but the wallet still enforces a 25 transaction chain limit. From running bitcoind -help-debug: ... -limitdescendantcount= Deprecated setting to not accept transactions if any ancestor would have or more in-mempool descendants (default: 25); replaced by cluster limits (see -limitclustercount) and only used by wallet for coin selection ...

If you want to test the mempool policy you can change -limitdescendantcount to a value larger than 64 and see if the 65th transaction is accepted (should not). Without changing the -limitdescendantcount option you will have to create the transaction chain without using the Bitcoin Core wallet.