I found the link on a forum thread from 2016 that had been "archived" but never deleted. The title was plain: Undertale_PSP_Port_v1.0_ISO_REPACK_Full.rar .
If you’re looking to play Undertale on a portable device, here are legitimate alternatives: Undertale Psp Iso Download REPACK
No. Undertale was originally developed using , an engine that does not natively support exporting to the PSP. Official console versions were later released for the PlayStation 4, PlayStation Vita, Nintendo Switch, and Xbox One. Any file claiming to be an "official" PSP ISO is likely a fan project or, in some cases, malicious software. Unofficial Fan Ports and Projects I found the link on a forum thread