If you try and check some blocks mined recently, you will still noticed that one or two blocks are mined by unknown miners. Those unknown miners are not mining pool but solo miners. It is not common but solo miners still mine blocks.
An example is block 883319 which was mined not up to a day ago.
The coinbase tag of that block suggests it belongs to Luxor Pool:
"wz Mined by Luxor Tech$W忮pmm*p7C>>Y"
Which means no all that we see as unknown miner are actually mined by a solo miner. Thanks for letting me know. Let me look for another latest one.
How about block Block 883181 which was mined 2 days ago. I think a solo miner mind the block.