Great video; Geoff Lindsey has a lot of good content, I strongly recommend his channel (in special his other video he mentions there). What I’m going to comment here is mostly additional trivia, as I watch it.

The diagram he shows around 2:00 should be accurate for East Midlands and London (both are roughly the basis of modern English pronunciation), but around 1400 I’d expect West Midlands and West Country to still have /y:/, perhaps even /ø:/ and /œ:/. AFAIK those front rounded vowels were merged into the unrounded set first in the East.

It also shows an interesting orthographic resource: ⟨ea oa⟩ for mid-open vowels. It made sense, since [ɛ ɔ] are midway between [e o] and [a].

Around 8:05, he mentions the long vowels progressively gliding. That change might look subtle, but it’s structurally important, it means the final loss of the long/short vowels contrast. Late Latin showed something similar in spirit, but the end result was simple vowels instead:

1. /e i/→/ɛ e/ (most Latin dialects; except in Africa and Sardinia)
2. /o u/→/ɔ o/ (most Latin dialects; except in Africa, Sardinia, Romania, and perhaps Andalusia)
3. /a: e: i: o: u:/→/a e i o u/ (all Latin dialects)

with the result being a 5~7 vowels system, /a (ɛ) e i (ɔ) o u/, and no vowel length.

12:30 or so, regarding natural classes, you can summarise the situation roughly like this:

• 1400 - length contrast is a fairly strong in the language; there are five short vowels, and seven long.
• 1700 - the so-called “tense/lax” pairs are fairly visible; the “lax” set (formerly the short set) is prone to centralise, and the “tense” set (formerly the long set) glides quite a bit
• 2000 - the tense/lax contrast is giving way to a simpler system, by reanalysing the tense vowels as their lax counterparts plus semivowel.

I can’t help but agree with his rant that analysing these vowels as long/short pairs by now is a bit silly.

18:30, talking about the phonetic representation of the vowels: it’s outside the scope of his video so he didn’t mention it, but another silly usage of the IPA to represent English is [ʌ]. That vowel is by no means as backed and closed as the symbol would imply, it’s more like [ɐ].