Functions like + will always check for overflow (except for float inputs). There's no way to globally disable this.
To allow overflow in a specific case, use functions like wrap-add.

Asserts can't be globally disabled.
If there are certain expensive asserts in your program that you might want to turn off, you will have to come up with a way to make them conditional.

() is the empty void value.

Parsing functions are in keen/parse.

There are often not string-specific functions when a generic collection function will work. In this case, you can use contains-seq to test for a substring.

log is allowed anywhere. This only writes to stderr and is not considered the program's output, so it isn't considered a true side effect. print is the program's actual output and so is considered a true side effect.

The standard library is used by all Keen programs, but the best interfaces to use depends on specific circumstances.
So instead, the standard library just defines global functions and leaves abstraction to the programmer.

To avoid confusion. It's unclear whether == on mutable values should compare the contents or the identity.
To compare the identity, use reference-equal. (This is unsafe in general, but can be trusted for mutable values.)
To compare the contents, convert to immutable values and compare those.

In Keen, an enum type is guaranteed to have one of the listed values. A flags type can have any combination of values.

If you mutated the object, its hash could change, making it unretrievable.
If you need to associate some data with mutable values (and can't just add it as a field), you could give the values an identity property and key the map by the identity.