I was a full-time student in spring of 2021, but withdrew for personal reasons. I am back in college now, but would I still be considered full-time for that year?

A full-time student is a student who is enrolled for the number of hours or courses that the school considers to be full-time attendance.

It depends on what you are tiring to do.  You (or your parents) can still claim an education  credit for the tuition you paid. You only need to be half time or more  and you do not have to have completed the term.

If you dropped out before May, you did not attend school for parts of 5 months and as such cannot be a "qualifying Child" dependent (if you are over 18).  But you may still be a "Qualifying Relative"

There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test. Only a QC qualifies a taxpayer for the Earned Income Credit and the Child Tax Credit. They are interrelated but the rules are different for each.

The support test is different for each type. The support test, for a QC, is only that the child didn't provide more than half his own support. The support test for a Qualifying Relative is that the taxpayer provided more than half the relative's support.

See full dependent rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Return/INF12139.html