For sure it is crossed out, because it doesn’t work on every site like this. The format is also there. Before update discourse 3.0 it was always without underline and not crossed.
This was a screenshot, before it went wrong, there you can see, it isn’t crossed out or underlined.
I use the red and green in my own topics and it was always “only coloured”. If you say, it has nothing to do with the update, okay, but I can tell you, it is since then.
The <ins> and <del> tags were never meant for coloring only: their purpose is to indicate an addition (insert) and a removal (delete). They are typically used to display computer code edits, but can also be used in a text like in a wiki revision to highlight what content was, e.g., deletedremoved and what was insertedadded. That’s why there’s only two colors btw!
Trusting their presentation was not the best option as it can vary over time to mean the same thing (add/remove). Discourse changed their presentation and may do so again in the future and technically it is correct for the purpose those tags exist.
If the purpose is to highlight something, use bold, italic, or even ## headers or > quotes. Even if the particular presentation for that may also change over time, it’s purpose is still to highlight text, so it won’t become unreadable.