Before someone says, "What about flatpak?"

I have personally been maintaining a flatpak for Vivaldi for quite some time.

https://flathub.org/apps/com.vivaldi.Vivaldi

The actual reason why is not yet official is that there are still some outstanding discussions about how secure the interprocess sandboxing of Chromium is when run underneath flatpak.

https://github.com/flathub/org.chromium.Chromium/issues/337#issuecomment-2308800825

TL;DR Both sandboxes (Chromium and Flatpak) are designed to work via "namespaces" but that does not work so well when nested. Thus it is not agreed how well Chromium processes (and hence websites) are seperate from each other, even if the browser is nicely seperated from other apps.

To be clear, that is not an issue on snap because its sandboxing uses different technologies. It is not a problem for Firefox on flatpak (because again their sandboxing works differently).

So out of caution (for now) the Vivaldi flatpak remains an unofficial, experiment. This is also probably why you do not find officially supported Edge, Opera, etc. on flathub but you do in the snapcraft store… despite the fact that in my personal opinion flatpak has the momentum.

Anyway, I just want to make clear. This is not some statement that Vivaldi loves snap and hates flatpak. If that were true I would not have been encouraged to look into flatpak packaging first. 😉