I looked at the verses from a couple of different bibles. The Amplified shows it as :
8 But at that previous time, when you had not come to be acquainted with and understand and know the true God, you [Gentiles] were in bondage to gods who by their very nature could not be gods at all [gods that really did not exist].
9 Now, however, that you have come to be acquainted with and understand and know [the true] God, or rather to be understood and known by God, how can you turn back again to the weak and beggarly and worthless elementary things [[c]of all religions before Christ came], whose slaves you once more want to become?
10 You observe [particular] days and months and seasons and years!
11 I am alarmed [about you], lest I have labored among and over you to no purpose and in vain.
Hopefully the Amplified is in error about Paul meaning ALL religions before Christ, as that would also include Judaism or at least the religions that were given to us by YHWH.
I think and hope that Paul is merely talking about the pagan/heathen practices and feasts.
The Amplified is of course a commentary. Most modern commentators are eager to group God's holy days and their observance into this category. No surprise there. That being said, Judaism, by the time of Christ, had corrupted the true religion given by God. So some of the non-scriptural, perhaps pagan elements, of Judaism could well have been referred to.