From the current Reddit User Agreement:

You retain any ownership rights you have in Your Content, but you grant Reddit the following license to use that Content:

When Your Content is created with or submitted to the Services, you grant us a worldwide, royalty-free, perpetual, irrevocable, non-exclusive, transferable, and sublicensable license to use, copy, modify, adapt, prepare derivative works of, distribute, store, perform, and display Your Content and any name, username, voice, or likeness provided in connection with Your Content in all media formats and channels now known or later developed anywhere in the world. This license includes the right for us to make Your Content available for syndication, broadcast, distribution, or publication by other companies, organizations, or individuals who partner with Reddit. You also agree that we may remove metadata associated with Your Content, and you irrevocably waive any claims and assertions of moral rights or attribution with respect to Your Content.

I found a historical version from 10 years ago and that version already had this:

you agree that by posting messages, uploading files, inputting data, or engaging in any other form of communication with or through the Website, you grant us a royalty-free, perpetual, non-exclusive, unrestricted, worldwide license to use, reproduce, modify, adapt, translate, enhance, transmit, distribute, publicly perform, display, or sublicense any such communication in any medium (now in existence or hereinafter developed) and for any purpose, including commercial purposes, and to authorize others to do so.

Haven’t dug up anything earlier than this, do you know of any?

Basically, you gave Reddit your approval long ago.