Valve have confirmed they're currently working on a level editor for Half-Life: Alyx which would consist of a subset of Source 2 development tools to allow modders to create their own maps for the game.
We knew one was likely coming, as Steam has listed the game includes a level editor since day one, and Valve's website states that "A set of Source 2 tools for building new levels will be available for the game". Valve's Jake Rodkin gave an update on the situation via Resetera however stating the team are now giving the new tools their full priority.
It’s the teams focus now that the game’s out. There will be more info soon but we’re working on it. Sorry I don’t have a date beyond that but I’m sure there will be more communication soon. Most of last week was spent on post-launch patching and tech support.
So while there's no firm date yet, I can't imagine we'll be waiting too long for these. Modders have already been hard at work on the game though regardless, and there's even a port of Half-Life 2 to the Alyx engine underway with impressive results.
While that mod converted maps from Half-Life 2 to the game, these new tools should make it much easier for modders to create their own, unique environments for the game.