Why it is not allowed any longer to solo mine directly using Bitcoin Core wallet?
Solo-mining is always allowed by Bitcoin Core. It'd be ironic if the official release of a Bitcoin client didn't have a solo-mining option.
Open your Bitcoin Core console and type “
generatetoaddress <nblocks> <address> <maxtries>”.
Example:
generatetoaddress 1 tb1qm87j39792m2afwduss7hfwqg3xxrrz2tz8yxrw 2000000000
You won't be able to solve a block, because the difficulty is incredibly high for an average solo-miner. Try
creating your own hard fork of Bitcoin and then use that for solo-mining, just to see that you're allowed to do that.