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In his recent video, Scott Manley mentions that after reaching orbit, the Space Shuttle would dump the excess fuel left in the External Tank before jettisoning it:
I did look at the video, thanks for starting it at the point of interest; what Manley says is simply wrong. On actual shuttle flights propellant was not dumped from the ET. Residual prop (both fuel and oxidizer) was dumped from the Orbiter after ET sep. He is describing this dump in a garbled, mistaken manner. There's no relation between this dump and the ascent design changes he's trying to describe either; the Orbiter lines always ended up full at MECO; anything else was off-nominal.
To be fair, the document he shows the cover of at 0:45 in the video does contain references to dumping prop from the ET, but it's from 1973 and bears little relation to the STS as actually operated. He also shows a timeline diagram that shows the prop dump before ET sep - in the actual shuttle, it happened after ET sep.
BTW, it was the ISS that came within a single vote of being cancelled, not the shuttle. I wouldn't suggest relying on Manley for credible info.
Source: Shuttle Ascent Checklist pg. 3-2
Early versions of the ET were fitted with a "tumble valve" that vented residual oxygen after separation; it was problematic and was deleted. Discussed briefly in this answer.
This answer goes through the ET sep sequence in detail; there was a failure scenario in which the crew would halt the sequence to let the ET blow down through a failed-open disconnect valve, but it never actually happened.
