The difficulty lies in the limited space between the starter and the clutch slave cylinder. You might be able to maneuver a socket and extension, possibly with a universal joint, to reach and remove the starter bolts. However, getting the starter itself out with the slave cylinder in place can be tricky.
Therefore, while technically possible for some, it's generally recommended to remove the slave cylinder. This significantly increases your chances of success and avoids the risk of damaging the slave cylinder or surrounding components during the starter replacement. Removing the slave cylinder only adds a small amount of extra work and is considered a relatively easy task.
In short, you *might* be able to do it without removing the slave cylinder, but doing so increases the chances of frustration, damage, and a longer repair time. Weigh the small extra effort of removing the slave cylinder against the potential hassles of trying to do it without.