Older Chinese folks (古人) love to use 昔人，but they never use 古人 in their poem or lyric。(OR can someone give me an example where 古人 was actually using 古人 in their writing?) For instance, a favorite poem from the tang dynasty: 昔人已乘黄鹤去，此地空余黄鹤楼。黄鹤一去不复返，白云千载空悠悠。On the other hand, newer Chinese folks now rarely use the word 昔; Instead, 古人 (昔人 old folks)，去年(昔年last year)，昨天(昔日yesterday)，前天 (昔日The day after yesterday)。Why do Chinese people change preference over time on word choices? And since 昔 means 从前，Technically, can we use 昔人 to replace 前妻/前女友？ OR does 昔人 have to be a 死人 (A already deceased person)？
never? you're kidding :) the term "古人" is also used in poem.
just a casual search:
in 通典·卷三十二 (end of p73, and start of 74):
next, 杜工部﹒ 吾宗
later, 孟宗獻 wrote
imo, characters are like fashion, "trendy" today, would out-of-date".
have fun :)
昔人 doesn't have to be 死人，lol. Honestly, I've never heard anyone calling his ex 昔人. But I think it's usable... May be too poetry for daily life...and so you might need to explain to people further, unless you use it in a poem. 故人，is a little more common than 昔人 in modern Chinese, but still, quite poetry. If you call your ex 一位故人， than probably you are implying that you two had a subtle past...