The maximum theoretical size for a zip archive is 16 exabytes (2^64 bytes). It's free if you have where to store it.

Should be doable on consumer hardware nowadays, if you cheat by using a file system that either supports sparse files (https://en.wikipedia.org/wiki/Sparse_file) or block-level deduplication (https://en.wikipedia.org/wiki/Data_deduplication). You may need to use raw block I/O to create such file, and there will be lots of duplicated content in the archive.

Also: how hard is that limit? ZIP archives have their TOC at the end of the file and allow for inserting ‘junk’ that is never referenced in the ZIP’s table of contents. Isn’t it possible to add such junk to make an archive go over that limit (assuming that your file system allows files larger than 2⁶⁴ bytes)?

The problem is once you zip them to full compression, you really can't use them ever again. That is unless you get the good ones that let you technically unzip without requiring destruction.