Is the CR line ending still prevelant on new applications or should it be considered legacy?
As usual it all depends on the environment your software is used in. If you're sure that all input will be [CR]LF delimited, then narrowing it down might work fine.
If it's guaranteed that all data will be produced by Mac Software less than ~5 years old, chances may be good. Just make sure it's never used with old(er) data. Data is not just produced by Macs, but many other systems as well.
My question is what impact would such obsoletion have?
As you already assume, it may break compatibility in some Situations. Chances for this to happen is all within the use szenarios of your library. Do you know them all?
Keep in mind, Data can come from many systems (not just ac or PC) and is of varying age. It's not uncommon to parse text created on some odd late 1970s system, as it my contain important business numbers.
So far for the 'how common part. Talking about design decisions of your new software is rather off-topic here, so readers may ignore the following.
Personally I would not only start to think about dropping either format. One of the most important aspects of data handling libraries is resilience against 'unusual' input.
Or, as the saying goes: Be gentle to any input and strict with output.
The other guideline here might be: Never optimize more than necessary.
Last but not least: Be prepared to enjoy many bug reports and feature request from upset users who happend to have an older file and do notunderstand why it can be opened in an editor but fails with your library.
Turning CR/LF sequences into line end tokens requires usually only a very simple state machine (*1), able to be implemented in less than a dozend assembler instructions. Not exactly a huge performance killer or complicating issue at all.
Now, if you're software design is already settled to a very pecific and rather limited approch, as the addendum suggest, then why ask at all?
*1 - Rewrote that line 3 times, hesitating to call it a state machine at all, as it's just a few IF clauses.