all versions below 4.6 are unsupported

Where do you see that? Per the policy, 4.5.2 is bound with the lifecycle policy of the parent operating system:

.NET Framework 4.5.2: Support for .NET 4.5.2 follows the lifecycle policy of the parent operating system. It is supported as a Windows component on the latest required operating system update for Windows Vista SP2, Windows 7 SP1, Windows Server 2008 SP2, Windows Server 2008 R2 SP1, Windows 8.1 Update, Windows Server 2012, and Windows Server 2012 R2.

Considering the product lifecycle of Windows 2012 R2 Standard, .NET 4.5.2 would be supported at least as far out as 2023, though mainstream support ends October of this year:

Anyway, we don't work for Microsoft so we're not bound by those dates, just the threshold for pain to support a particular target. We can certainly drop 4.5.1:

.NET Framework 4.5.1: Support for .NET Framework 4.5.1 on all operating systems ended on January 12, 2016.

Any reason to skip 4.5.2 and bump to 4.6 here?

all versions below 4.6 are unsupported

Where do you see that? Per the policy, 4.5.2 is bound with the lifecycle policy of the parent operating system

Oh dear. Embarrassing. It appears I need glasses 👓 .

Any reason to skip 4.5.2 and bump to 4.6 here?

No, not at all. I'll rework to use 4.5.2

Anyway, we don't work for Microsoft so we're not bound by those dates, just the threshold for pain to support a particular target.

We are not bound, I agree. Yet we don't need to take on the pain of supporting old stuff for too long. I find that the upstream support timeframes are a good Schelling point for providing upper limits for target support.