Ok, to maybe give some final information on the discussion, because it is admittedly complex and not really manageable by people not speaking German:
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Since 2017 there is officially an uppercase version of the ß (after a over 100 years long debate). But the most common way is still to write it as SS. The use of the uppercase ẞ is allowed but optional. In my experience so far its usage is very rare and to most people it looks odd (e.g. at least in the font I see here in “DIE GRÖẞTEN ERFOLGE” the ẞ still looks out of place and not really fitting the rest).
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If this was a release from Switzerland targeted at a Swiss market it probably would be a lowercase “ss”. The Swiss got rid of the ß long time ago. They will sometimes still use it if targeting a broader German speaking audience. For MB releases this could be hard to decide, but likely the very specific Swiss only releases are rare.
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You might also find ß replaced by ss in some lowercase texts as a result of technical limitations (e.g. pure ASCII encoding of text).
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In older texts uppercase ß might also show up as SZ. Probably not so much of an issue for MusicBrainz.
If a release uses normal capitalization for text on the cover but uses “ss” instead of “ß” I would also go for the “ss”. If the uppercase text uses an uppercase ẞ it’s an easy decision, use lower case ß. Otherwise when lower casing upper cased text replacing “SS” with “ß” is usually the right thing to do, if the lower cased word would be written with an “ß”. If in doubt for a specific release I think both ways would be fine, nobody will get stoned because of this 