AFAIK Uniswap doesn't have an upgradeability proxy, they just deploy new versions and the old ones stay up. That's only true if you aren't using a L2 though, all the current L2 implementations are upgradable.
Looks like you're right - Uniswap v1 is still usable, they had 'proxy' contracts that could redirect users to more recent versions, but you can, if you choose, interact directly with the original version - it wasn't turned off. However, since the liquidity all moved into the later versions, there's only about 1/1000th of the trading left in the original. (and you get to keep/be exploited by all the unpatched bugs, yay!)
Uniswap v1 volume: $144k, v2: $111,784k