That’s not how software development works. Something as simple as underscores in usernames was announced in October of 2015 and shipped in February of 2016 (5 months), and this only included implementation/testing and not design.
More trade slots also needs to be researched/design so it doesn’t have negative consequences on the platform – for instance, will more trade slots save more user items that are lost through multi-trade scams than it loses through stealing someone’s entire inventory at once? Do users really need to be able to trade their entire inventory in a single trade, or is just bumping up the number of trade slots (e.g. 10) enough for legitimate trading needs? And when we increase the number of trade slots, how do we change the interface? Do we add a horizontal scrollbar? Add another row of slots?
Design, implementation, and testing are not fast processes – especially on a scale where you have to support a userbase the size of ROBLOX’s. This is not something that could be knocked out in an afternoon. It is likely slated for whenever the trading interface is overhauled, but there are a lot of more prominent pages on that list as well (groups, “community” forums/equivalent, develop page, etc). There are a lot of other users’ needs that need to be addressed as well – not just traders’.