Some ways to try to revert it (some have already been mentioned)

👉 Go to this page and check if you’re able to opt out of the “Redesigned Watch Page” (source)

👉 Install a userscript addon like Violentmonkey, and then use a script like Classic Youtube Layout (which is based on this script which was outdated in regards to the related videos sidebar)

• If you get an error about the @match line of the script then click on allow edits and change it from *.youtube.com/* to *://*.youtube.com/*, or install the script as it is and then put *://*.youtube.com/* in the “@match rules” line in the settings tab of the script
• There’s also this script that you would need to add its code manually to the userscript addon by clicking on the new script icon (see the author’s post for more details)

👉 Use uBlock Origin filters like these or these

• Note that youtube.com##+js(set, yt.config_.EXPERIMENT_FLAGS.kevlar_watch_grid, false) should be enough to change the UI back for the most part

👉 Use this userscript and Stylus style

👉 The addon Enhancer for YouTube (Firefox / Chrome) also reverts the UI, at least partially

👉 (might only work some times) Use the addon CustomTube - Firefox / Chrome (the default UI is from 2017 but you can choose 2021 in the settings. It’s not exactly like the one from 2023 but it’s close enough)

• note that it removes some buttons like Join by default so go over the settings if relevant

If you also want to change the number of videos per row in the subscription page and channels’ videos page, then you can add this uBlock Origin code to the “My filters” tab (at the end click “Apply changes”, or Ctrl+S) (this is for 6 items per row but you can change the number in the first 2 line as needed) (source 1 / source 2)

youtube.com##ytd-rich-grid-renderer:style(--ytd-rich-grid-items-per-row: 6 !important;)
youtube.com##ytd-rich-grid-renderer:style(--ytd-rich-grid-posts-per-row: 6 !important;)
youtube.com##ytd-two-column-browse-results-renderer.grid-6-columns:style(width: 100% !important;)
youtube.com##ytd-rich-grid-row, #contents.ytd-rich-grid-row:style(display:contents !important;)
youtube.com##ytd-two-column-browse-results-renderer.grid:not(.grid-disabled):style(max-width: 100% !important;)

Sorry, it was unclear in my comment. By “it’s actually part of the Mozilla Corporation” I was referring to Firefox, not Mozilla Foundation

for anybody that wants to disable it, go to the settings and search for “Allow websites to perform privacy-preserving ad measurement”

(or through the dom.private-attribution.submission.enabled flag in about:config)

https://support.mozilla.org/en-US/kb/privacy-preserving-attribution

Just adding that as I understand this, donations to the Mozilla Foundation cannot go towards Firefox, because it’s [edit: Firefox is] actually part of the Mozilla Corporation. To help with funding Firefox people can consider purchasing the Corporation’s other products (VPN/Relay/Monitor), or purchasing merch.

See more here on the AMA on Reddit, and this thread

On the humorous side:

• My Brother, My Brother And Me - 3 brothers giving not-so-serious advice to listeners, reacting to questions from Yahoo Answers (when that was a thing), and overall being funny/silly
• My Dad Wrote A Porno - 3 people reacting to an “erotica” book series that one of the guys’ dad published (I would say the books are maybe only 5%-20% explicit, depending on the chapter) (some of the characters’ voices can be annoying/grating, and the narrator tends to repeat sentences after they react to them which can be annoying as well)
• No Such Thing As A Fish - the behind-the-scenes staff of the show QI bring up interesting facts and tidbits from history/nature/etc. (each episode is split into 4 parts where each member brings up a fact and the others react to it and bring up related facts)
• If I Were You - Jake & Amir from CollegeHumor giving advice to listeners (mostly in a sarcastic/tongue-in-cheek way but sometimes genuinely), mostly about relationships/dating
• SmartLess - Jason Bateman and Will Arnett (Arrested Development) and Sean Hayes (Will & Grace) interview a famous person each episode where only one of the hosts known who it is beforehand (it gets better after the first few episodes, though some conversations are less funny/entertaining than others)
• Office Ladies - Jenna and Angela from The Office (US) reacting to each episode of the show and bringing up behind-the-scenes stuff (some of episodes include interviews with other cast members/staff)