Conventional subs make superior attack subs, and when a sub yet again "sinks" an aircraft carrier in a training exercise its a (often German-built) non-nuclear sub.
Nuclear subs of course excel in staying hidden for months with multiple nuclear missiles. No conventional sub could ever match them at that. But that's not the only thing subs do.
Credit to the Swedes for repeatedly schooling the USN with their Gotland-class AIP submarines.
Describing nuclear as superior betrays a lack of post cold war updates. Nobody serious maintains this position, except in the very narrow use case of global navigation and ICBM launches.
Here's an article about a new lithium-ion one:
https://www.thedrive.com/the-war-zone/37083/japan-just-launc...
South Korea just launched the ROKS Ahn Mu, the second sub in its class, in November.
Australia's Attack class is beginning construction, with 12 subs to be delivered. They are planned to be in commission from 2030-2070.
That being said, are there ANY countries still running diesel subs? I can't imagine what the point would be in modern times. Not everyone can afford nuclear subs, but on the other hand you'd be crushed so fast by a nuclear navy I can't imagine why'd you even bother trying.