[racket] use of SRFIs in Racket

Good evening all,

This might be a silly question -- but what is the official status of the 
included SRFIs in Racket as they relate to #lang racket?

I am working on my first Racket program and I need to be able to test if 
a given string is present inside another string.

For example: I need to be able to ask if the string "Hello how are you?" 
contains the word "how".

In other words, I need a function like string-contains, as found in 
SRFI-13.  However, I am unclear on the breakdown between racket & 
racket/base and the SRFIs.  I know that they are there from an 
historical perspective (PLT Scheme) but are they something we should be 
using in new *Racket* code?

If so, are they planned to be supported long-term as standard libraries 
of Racket? Or are there any plans to eventually integrate some of what 
they provide into Racket itself? (Do these two questions even apply, or 
are SRFIs such a part of racket as to make it nonsensical?)

If not, what is the suggested approach in this circumstance?  I'd rather 
not re-invent the wheel, but neither do I necessarily want to depend on 
these libraries that are hidden away in the documentation when writing 
idiomatic Racket if they might disappear someday. Forgive me if I'm 
missing some other trivial way to test for this; I'm coming from a Ruby 
background and still learning Racket.

And, as a question of semantics, I am referring to the #lang racket/base 
and #lang racket sub-family of languages; what I, as a newcomer, see as 
the core and standard library of Racket (since Racket can be so many 
different wonderful things!). I'm not sure that's an entirely accurate 
way to think about it, but again I betray my Ruby background.

Thanks for reading,

Cheers,
Edward