Will Proton VPN block website ads in IOS browsers (Chrome, Safari, etc)?

I can see that the free version doesn’t and I’m thinking of purchasing Proton Plus.

Yes, as long as Proton has that specific tracker / ad listed in their list.

even its NetShield, won’t block any ads in Safari, including ads in YouTube, LinkedIn, Reddit, X e.tc

DNS based ads can be blocked with netshield on and the setting “malware & trackers”.
Youtube ads cannot be blocked by any dns blocking solution, as the ads are from the same URL as the videostream.

You can verify that by checking the results of the two pages here, with Netshield off and Netshield on:

https://test.adminforge.de/adblock.html