Hello !
I needed this feature too last year, my browser computer turn public ( friends and surfcouching guests can also use the low-watt consumption webrowser computer but my working computer is locked - my work need a powerfull processor and a lot of ram, and it's off-grid near alltime to keep it safe from virus without an antivirus ).
I am using some portable applications only to keep clean windows register on the low consumption one.
So because it's impossible to put a password on Opera, I've switched to firefox portable because it got this feature.
On this portable firefox without the password it's impossible to get the stored password of facebook, insta & ect, but you can still use the web browser.
I am on this post after a search on duckduckgo " how to put a password on opera".
If some opera DEV program it ( I don't know if chromium base web explorer can abble this) post here, I'll get an email alert and switch back to opera.
Anyway best regards to the DEV to their nice work, I still love Opera !