"An older steel bike" could be anything from Schwinn Varsity from the 70s that weighs about 40 lbs to something like (sticking with Schwinn) a Circuit from the late 80s that weighs 21 lbs. The former would hold you back, the latter much less so. But weight isn't the only thing that could hold you back. If you live in a hilly area, you might find yourself avoiding climbs because the gearing on bikes that old is often quite limited. If the hubs and BB have not been serviced, you might be fighting more friction than you need to. If your position on the bike is less than optimal, you might find that unnecessary discomfort holds you back.

That other question I'd ask, though, is "hold you back" from what? If you just want a workout, any bike can do that for you. If you want to go farther and faster, a better bike will probably help. If you want to do group rides, I'd suggest that "an older steel bike" may have less than stellar braking, downtube shifters vs brifters, and, again, suboptimal gearing. I have, and love, and love to ride a couple older steel bikes with those very less than stellar brakes, downtube shifters, and suboptimal gearing. I'm happy to take any of them for a 60 mile Sunday ride, and I'll flog them as hard as my aging legs allow. But I would not attempt a fast group ride on any of them.