Can FTRL be applied on linear least squares? or is it just for logistic regression models?
As we can see, FTRL is nothing to do with our model, and it only serves as a way of approximating the optimal solution , just as SGD does.

Logloss is just a kind of loss pattern in classification problem,and there are other alternatives for it , such as ExpLoss. Least-squres loss is one of all loss patterns in regression problem.

The reason why FTRL was mentioned everywhere ,which mainly treated Logloss as a example, is that it was often used in classification problem , such as CTR model in online advertisement scenario.

Hopes this can helps you ,and good luck !