Yes, it is true. Expressing uncertainty or speculation by ましょう is getting less and less common those days. Some old people still use it that way even today, like: 明日は雨が降りましょうか？(Will be rain tomorrow?) However, most people use でしょう instead of ましょう to mean that: 明日は雨が降るでしょうか？(Will be rain tomorrow?) Today, ましょう is usually used to express intention or suggestion. わたしが行きましょう (I will go) わたしが行きましょうか (Shall I go?) いっしょに行きましょう (Let us go)

I think there isn't a particular difference between those two expressions; however, desuka is more grammatically correct whereas deshooka is more conversational expression.