Please be assured that new features in XKit Rewritten do not enable themselves. The only circumstance in which anything will enable itself is if we replace a feature's option with something else (e.g. moving an option from Tweaks to No Recommended, or splitting one option into two that when combined do the same thing as the old option).

Panorama is not a new feature, but it has been broken for around two years as a result of Tumblr's new three-column site layout. What's likely is that you enabled the feature before or during this time, and now that it's fixed, it's obeying your pre-existing configuration and "suddenly" activating as a result.

If you have any recent configuration backups, you should be able to see Panorama in the list of enabled features there to corroborate what I've stated.

Hi there! Is this still occurring for you? If so, please open a GitHub issue so we can help diagnose this: https://github.com/AprilSylph/XKit-Rewritten/issues/new/choose

Check out Palettes for Tumblr! It does the job even better than Old Blue.
https://addons.mozilla.org/en-GB/firefox/addon/palettes-for-tumblr/

I agree the lack of cosmetic filtering by default is a problem, so I've contributed updated filters to EasyList to hide sponsored post containers, the sponsored sidebar section, and any takeover banners; this is awaiting review by EasyList maintainers, but in the meantime you can hide sponsored post containers by enabling AdGuard Base in uBlock Origin's filter lists.

EDIT: Due to the recent frequency of Tumblr regenerating their classnames, Anti-Capitalism has now been added as a feature.

This is now a thing! You can enable tag suggestions in Quick Reblog -> "Suggest tags from the post being reblogged", which suggests all tags present on the post you're reblogging, as well as the post author, original post author, and determined post type.