The information for each telescope is shown in the Deep Sky Menu (see red circle left menu). Then scroll down and you will see the Minimum target elevation (highlighted here in yellow at the bottom middle).

Most of the telescopes at New Mexico and Australia can get to within 25-30 degrees of the horizon. T11  can get to within 15 degrees.

The telescopes at Nerpio Spain can get to within 35 degrees of the horizon.

The limiting factors are Observatory wall heights and horizon obstructions. 

Imaging at these low altitudes is not recommended due to atmospheric distortions, guidance and focus problems.

Sky clarity and blue light extinction may also become issues at low angles, but if your a science user chasing a low target or comet this is not an issue.

Imaging is at its best quality at angles of 60 degrees and above from the horizon and away from a moon.