No. Tera will make Venusaur pure grass so it won't get stab on poison anymore

Edit: I googled it and I was wrong. You will still get stab, however you're grass moves will be powered up. I was thinking it changed to mono, but what it really does is it your pokemon becomes that type, while still retaining stab on it original typing. It's like adding another typing to your Pokémon.

Poison should still have stab even when terastelising, from what I know.

It won’t get Tera stab but its regular stab won’t change