I haven’t seen too many examples where a community reactivated a double-digit % of their former members.
Don’t waste your time trying to reactivate former members. They’ve made the decision not to participate. It’s far harder to change someone’s mind than to persuade someone who hasn’t made up their minds. Focus on the latter group.
It’s easier to attract new members than reactivate old members.
Think about it yourself. How many communities did you return to after a long time away?
You can revive and revitalise an old community, but you won’t do it with the old members. You’re going to do it by taking whatever embers remain from the old community and creating something slightly different.
Every second you spend trying to reactivate former members is time you can’t spend keeping your existing members (or attracting new ones). Your former members are the hardest group to convert into active members.