I see a lot of peoples are lost and did not understand this chapter so let me break it down for you:
I can explain slightly better.
The .5 chapters, outside the first one, had supernatural elements, in the 20.5 he found some glasses that made him see then as if they were naked except Yuyami that then took then, she been present in most of then as well.
They long hinted she was unusual, her name literally means
Twilight Beneath the Snow that might not be
that usual but it was another hint, this is the .5 chapter that is more serious about things instead of the usual fanservice ones with 4th wall breaking.
As for that last page ...
I hope people do realize these are volume scans, not Jump+ as this series was only published in Jump+, looking at the dates the series ended in 13 September 2019 with Volume 7 (this one) being published in 4 September 2019, that is about a week so the author knew the series was going to be finished by the time Volume 7 was going to be printed, thus that page.
Edit: Also no, the authors started doing this with volume 2 extra chapter and this was never fully intended to be part of the main series that for most part lacked any kind of supernatural, I will say this will be brought back in the epilogue chapter ... that I am looking forward to.