I dont know where I have it from (I think somone on reddit posted it), but I use this in the uBlock Origin Filter since 8 month and it removes all my shorts from the subscription:
www.youtube.com##ytd-guide-renderer a.yt-simple-endpoint path[d^="M10 14.65v-5.3L15 12l-5 2.65zm7.77-4.33"]:upward(ytd-guide-entry-renderer)
www.youtube.com##ytd-mini-guide-renderer a.yt-simple-endpoint path[d^="M10 14.65v-5.3L15 12l-5 2.65zm7.77-4.33"]:upward(ytd-mini-guide-entry-renderer)
www.youtube.com##ytd-browse #dismissible ytd-rich-grid-slim-media[is-short]:upward(ytd-rich-section-renderer)
www.youtube.com##ytd-browse[page-subtype="home"] .ytd-thumbnail[href^="/shorts/"]:upward(ytd-rich-item-renderer)
www.youtube.com##ytd-browse[page-subtype="subscriptions"] .ytd-thumbnail[href^="/shorts/"]:upward(ytd-grid-video-renderer,ytd-rich-item-renderer)
www.youtube.com##ytd-search .ytd-thumbnail[href^="/shorts/"]:upward(ytd-video-renderer)
www.youtube.com##ytd-watch-next-secondary-results-renderer .ytd-thumbnail[href^="/shorts/"]:upward(ytd-compact-video-renderer,ytd-shelf-renderer)
www.youtube.com##ytd-watch-next-secondary-results-renderer ytd-reel-shelf-renderer
www.youtube.com##ytd-browse[page-subtype="subscriptions"] ytd-video-renderer .ytd-thumbnail[href^="/shorts/"]:upward(ytd-item-section-renderer)
www.youtube.com##ytd-browse[page-subtype="channels"] #contents.ytd-reel-shelf-renderer:upward(ytd-item-section-renderer)
www.youtube.com##ytd-browse[page-subtype="trending"] .ytd-thumbnail[href^="/shorts/"]:upward(ytd-video-renderer)
www.youtube.com##ytd-search #contents ytd-reel-shelf-renderer
m.youtube.com##ytm-reel-shelf-renderer
m.youtube.com##ytm-pivot-bar-renderer div.pivot-shorts:upward(ytm-pivot-bar-item-renderer)
m.youtube.com##ytm-browse ytm-item-section-renderer ytm-thumbnail-overlay-time-status-renderer[data-style="SHORTS"]:upward(ytm-video-with-context-renderer)
m.youtube.com##ytm-browse ytm-item-section-renderer ytm-thumbnail-overlay-time-status-renderer[data-style="SHORTS"]:upward(ytm-compact-video-renderer)
m.youtube.com##ytm-search ytm-thumbnail-overlay-time-status-renderer[data-style="SHORTS"]:upward(ytm-compact-video-renderer)
m.youtube.com##ytm-single-column-watch-next-results-renderer ytm-thumbnail-overlay-time-status-renderer span:has-text(/^(0:\d\d|1:0\d)$/):upward(ytm-video-with-context-renderer)
It will sure break in a couple of month again. It did 8 Month ago and I am sure YT will change the code again at some point.
I do not trust having to many FF Extensions installed from some rando from the Internet, so I like this solution better.