The domain definitely still resolves:

$ dig ftp.boulder.nist.gov
<…snipped…>
;; ANSWER SECTION:
ftp.boulder.nist.gov. 300 IN A 132.163.4.45
<…snipped…>

An ftp client can still connect to this server and the relevant file is still present:

$ ftp ftp.boulder.nist.gov 
Trying 132.163.4.45:21 ...
Connected to ftp.boulder.nist.gov.
220 ProFTPD Server (NIST Time/Frequency Division FTP Server) [::ffff:10.88.0.2]
Name (ftp.boulder.nist.gov:root): anonymous
331 Anonymous login ok, send your complete email address as your password
Password:
<…snipped…>
230 Anonymous access granted, restrictions apply
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls pub/time/leap-seconds.list
229 Entering Extended Passive Mode (|||20091|)
150 Opening ASCII mode data connection for file list
-rwxr-xr-x   1 ftp      ftp         10921 Feb  7  2024 pub/time/leap-seconds.list
226 Transfer complete

Indeed, you can generally skip these steps by using something like `curl`:

curl ftp://ftp.boulder.nist.gov/pub/time/leap-seconds.list > leap-seconds-NIST.list

However, I note that the version of the file which is present on the NIST server is dated 2024-02-07 and has an expiration date of 2024-12-28.  It's not clear whether this is just another temporary lapse on NIST's end or whether this points to a more recent deprecation of service of some sort.  For what it's worth, other files on that server, namely in the wwvb directory, do appear to still be receiving (automated) updates.

Perhaps part of the confusion here is that a URL with the ftp:// protocol can no longer be naïvely pasted into a modern web browser.  As FTP usage declined steeply over the last decade or two, browsers have broadly dropped their once-native support for the FTP protocol.  For example, Firefox 90 and Chrome 95 completely removed this functionality back in July and October 2021, respectively.  In both cases, such functionality had even by that point already been deprecated, disabled-by-default, and on the chopping block for several years: