Reminds me of SAE AS83519/1 series shield terminators, which contain a Sn63 solder ring preform and flux in the middle of a translucent PVDF heat shrink insulation that seals on both ends on application.

For conductors with robust enough insulation (i.e. at least 105 degC rated), and assuming you're not beholden to RoHS, they make for quick, permanent, production-worthy splicing with just a heat gun...and you can actually buy them today.