The big internet corporations (like Google, Meta, Amazon) have independently developed their own smearing behavior, some even iterated over multiple smearing patterns. If you search for <corporation> leap smear with an internet search engine of your choice, you will find blog posts and documentation where they explain their respective approach. I suggest you research this for any upstream server you intend to use. A quick summary of the ones I came up with spontaneously:

• Google, Amazon: Smeared from noon to noon around the leap with 1/86400 of a second added to each second
• Meta: Smeared from the leap until 17 hours later in a non-linear curve
• Cloudflare: Standard-compliant insertion/deletion of a leap second and propagation of the leap indicator (that’s why they are allowed in the pool)
• Microsoft: No idea what software their time servers are running, but the Windows Time Service included in Windows machines does not handle leap seconds and just resyncs after one happens

All of this is best practice information and much less relevant since the decision was made to do away with leap seconds in the foreseeable future. //

Apple also offers good collection of stratum 1 servers that doesn’t use leap second smearing. Someone compiled a (non-definitive) list of them so you can see if they are worth using for your location:

pool time.apple.com