It partially depends on your own goals.
Short answer I would give you is: Yes. You are limiting yourself by only playing one role. One of the fastest ways to boost your ability to apply knowledge in the game is to have a good understanding of the fundamental skills required to play and needs required to succeed in all roles in the game. (And you actually really, really want to learn how to jungle to fully appreciate objective-based gameplay in league. It's the most important role any other main needs to understand.)
However...
To what extent this is useful to you to keep sharp on really depends on how much time and effort you want to put into the game. It's hard to keep up on the knowledge advantage for all roles if you only put in 2 or 3 games a week, for example. Support is also currently the easiest role you can get away with only playing right now because you will almost never be forced to play a different role if you don't want to.
Personally, I've been playing nothing but support this season as kind of a personal challenge (not for the last month+ cause of computer issues I'm hoping to fix soon), but I do think from just the simple metric of "how enjoyable is the game" it prevents things from getting overly tedious if you branch out into different territory now and again. There are a lot of fun champions to play in every role and you miss out on the different experiences available if you're stuck in a rut of your own making.
Really, I tend to think you should just boil down your decision making to this basic flowchart in regards to the game:
Q: Is the thing fun to do?
--> Yes --> Do the thing
-->No --> Try a different thing
If you just don't want to branch out because you're scared or uncomfortable with your performance elsewhere, it's okay to take your time, but don't let be the thing that limits your experiences.