I understand all the tools and services of D&D beyond are 5th edition, I was curious if talking about older editions of the game on these forums is okay or not.

For the record I first started playing when 5th edition was first released by going to an adventurer's league hosted by my local game shop. From there I met up with another group of players and we eventually moved to playing on our own outside of the game shop because the day of the week adventurer's league was held at the game shop no longer worked with some of their schedules and we decided to leave the league and play on our own time rather than split up the group. This meant we needed a new DM and happily someone in our regular group had experience DMing, but they were way more familiar with the older 3.5 edition so we switched to playing that edition for a while and then have settled on a system of alternating campaigns where, depending on who is DMing for the week we move between 3.5 and 5th edition.

Once D&D Beyond is past the workload that comes with the official launch, is this site opposed to adding one subforum dedicated to discussing older editions of D&D, where people can talk rules, ask questions and wax nostalgic about campaigns long past? (Just to make myself clear, I'm not asking for D&D Beyond to support older editions, I was just wondering if this site would be opposed to having a place in their forums where people can talk about older editions?)

I don't know if they'd be willing to consider subforums, but I do know that there's no reason not to talk about older editions if for some reason you want to. 

Try https://www.dndbeyond.com/forums/d-d-beyond-general/general-discussion

 I would add to the above by suggesting that you start a thread on it.  If it gains enough steam and conversation, this will greatly yield to discussions on creating separate forum/sub-forum.