Does SQL Server's is_nullable mean anything at all? SQL Server has a type flag called is_nullable you can see it on sys.types. Currently, (SQL Server 2019) it's only set to FALSE two types, sys.timestamp and sys.sysname. On sys.timestamp this type seems to accept null. On sys.sysname it does not. Is there any explanation of this behavior?
You can see these types with
SELECT * FROM sys.types WHERE is_nullable = 0;  
SELECT TYPEPROPERTY('timestamp', 'AllowsNull');  -- returns 0

And you can use it like this,
CREATE TABLE foo ( a sys.timestamp );
INSERT INTO foo (a) VALUES ( null );

Note this won't work if you use sys.sysname

Msg 515 Level 16 State 2 Line 1
Cannot insert the value NULL into column 'a', table 'dbo.foo'; column does not allow nulls. INSERT fails.
Msg 3621 Level 0 State 0 Line 1
The statement has been terminated.


It also will not work if you create a user-defined data type which is NOT NULL,
CREATE TYPE bar FROM int NOT NULL;

Is this just a hint for the user?
The answer to your question is yes.
That column specifies the default nullability of a data type, which can be important if the user doesn't express a preference e.g. in a CREATE TABLE statement with an explicit NULL or NOT NULL specification. Sadly, that information is only present in the documentation for the old compatibility view sys.systypes:

allownulls (bit)
Indicates the default nullability for this data type. This default value is overridden by if nullability is specified by using CREATE TABLE or ALTER TABLE.|

For example, one can override the default NOT NULL behaviour of the sysname type:
DECLARE @T table (s sysname NULL);
INSERT @T (s) VALUES (NULL);
SELECT s FROM @T AS T;





s




NULL




The timestamp type (preferred name rowversion) has additional special behaviours due to its intended use to detect row changes. See this other answer for some more details. You can still override its nullability in the same way, but it is not possible to actually store a NULL for a timestamp/rowversion attribute.
There are several complex layers to determining attribute nullability. Rather than repeat some of that detail here, I refer you to Why you Should Always Specify Whether a Column Accepts Nulls by Phil Factor for background information and additional reading.