[_mlbt]

I have a feeling that much of it was translated to C or C++ at some point for portability and maintainability reasons. There are several automated Pascal to C translators out there, such as the following...

http://users.fred.net/tds/lab/p2c/

Also the languages are similar enough that a programmer with knowledge of both could translate it manually without too much difficulty.

[dunham]

Typically TeX is translated from Pascal to C too, via web2c.

But there also is a Pascal to WASM compiler out there, which was written specifically for TeX:

https://github.com/kisonecat/web2js

TeX itself is only about 500kb of wasm, uncompressed, but the memory images with LaTeX loaded are quite a bit larger.