Unposted transactions in fixed assets is described as a transaction that does not need to be posted. This status can be caused by migrated transactions as @AmaanFazeer11 noted or by transactions that only interacted with non-posting books. This is likely why you see more recent unposted transactions.
Here is a prior suggestion I have made which might provide clarity on the topic:
[Unposted FA Transactions are] a continued concern and requires constant training caused by the presence of Unposted transactions in the system, which users are unable to post or delete. This confusion often stems from transactions originating either from data migration or involving non-posting books.
To address this, I propose a change in the status terminology.
Transactions generated during a period when 'Update GL' is deactivated in FA Preferences should be labeled as 'Migrated'. Furthermore, for transactions that are finalized [but not in need of further processing, such as those that do not involve posting books], the status should be changed to 'Complete' (alternatively, 'Released' could also be considered). This modification will enhance clarity and reduce the need for repetitive training.
Hope this helps!