Where to get old LineageOS ROMs? I have a few old devices which I want to put on LineageOS. This time it's about a Samsung Galaxy S4 mini (GT-I9195, serranoltexx). But the official LineageOS website shows them as not supported anymore. 

You basically are required to build the ROM for yourself, but this is beyond my knowledge and commitment.

I already have flashed successfully exactly such a device with LineageOS in the past, but do not have the ZIP around anymore.

Why does LineageOS not keep the latest versions of their ROM's around for download? Where to get a trustworthy copy?

Note: The same happened with cyanogenmod earlier. The did not store older ROM's too.

Note2: These devices were still on sale on recycling shops recently.
To answer the question (getting old builds) directly - there are people who foresaw the need and stashed some of the builds elsewhere: 1, 2.

As for the reason to it, it's right in their FAQ. The main reason would be to save bandwidth/storage, since the project is a non-profit.