Read gcp's policy and they say preview periods can last around 6 months, and I'm not sure when alloydb preview started.
But even if there's a free period, it'd be complicated to develop stuff around the DB for free, just to turn into 100s of dollars after 6 months, that's not something we want to see happening. So an indefinite free tier with limited resources would be better. Like aws lambda 1M or firebase function 2M request free tier.
https://cloud.google.com/alloydb/pricing#fair-usage-limits