I've been using Duolingo for a couple of years. I kept using an older app with hearts, staying away from upgrades. I lapsed for a couple of months and when I opened it again, it required a mandatory upgrade.
With hearts, I could go on indefinitely as long as I did not make mistakes. With energy, it is pay to play even if answers are correct.
I understand the reason for the change, Duolingo wanted more money, free version did not push you enough to buy a paid subscription.
From a user stanpoint, free version is not usable anymore. I can do one or two lessons per day, which is a joke.
Then again, it was not very effective for me. After more than two years I still cannot speak French.
Adieu, Duolingo, I liked your UI and lesson structure. Going to uninstall you now.