@email@example.com don’t worry, this is not a basic question!
If a part goes obsolete, you can use the Mouser parametric search to find replacements - here we need a 74HC595 in SOIC16 format - there are quite a few different products available - the 74HC series probably won’t be obsolete for the next decades to come 🙂.
Here’s the best replacement: 595-SN74HC595DWR
Note that the part number is nearly identical, it’s even a part from TI - only the letters at the end are different, which indicate a different packing method.
I’ve also updated it directly within the BOM, so after a browser page refresh, it should appear there. It also seems that all parts are currently available for SEQ v4+, which is pretty good! 🙂
Best regards and enjoy building!