2 icons for "boundaries"

border-corners

border-sides

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All under MIT or ISC license - free for your personal and commercial projects.

Phosphor

1,512 icons MIT

Simple Icons

3,397 icons CC0 1.0

Lucide

1,668 icons ISC

Phosphor Filled

1,512 icons MIT

Feather

Heroicons

Tabler

4,985 icons MIT

Meteocons