Requested feature overview description.
Add a new sorting/order option for Posts which would return the list sorted based on the time a specific tag was added.

I know this has been suggested before but I'm including more info. My thread is better, ok?

Why would it be useful?
Let's say you often check out a tag. You know there's 999 posts that have this tag. Then suddenly this number changes to 1000. How will you find the post that had this tag added to it? Currently this is MUDA.
I know it can be solved by using a set, but that's really annoying as you have to keep updating that set, plus it's a set spam

So yeah, this feature would be very useful for finding older posts that had tags added to it recently.

In a duplicate thread a certain parasprite said that

added tags aren't dated in a way that makes them easily searchable

Based on this, I'm assuming the tags currently have a timestamp of when they were added, but this timestamp is in a string format. This isn't a problem tho.

Anyway, possible solution:
1. Convert the timeadded(?) column in the tags_posts(?) datatable from text to one of the time offset types, such as "datetimeoffset" or "timestamp", which are represented as a large number.
2. Add an index on this column.
Actually, forget the first two steps. You'd get the right order simply by selecting the default order from tags_posts(?) datatable. No database changes required at all.
3. Add a new sort option, order:tag
Note that order:tag implicitly includes the said tag, so it would actually be processed as tag order:tag
Also note that other order: search options take priority over order:tag, so if you'd use order:score, the posts would get ordered by score rather than being ordered by when a tag score was added. In this case, there's two tags which have notable post counts, specifically, portrait and landscape. To also allow ordering by when those two tags were added, I would suggest renaming order:portrait into order:heightratio and order:landscape into order:widthratio.

What part(s) of the site page(s) are affected?
Posts area.