Good start, although your info is a little outdated.
The main way to achieve silencing is through dampening inside the switch, so-called ‘silent switches.’ The use of O-rings, by contrast, is not preferred, as it can increase mushiness.
Lube is typically applied to silent switches as well, although it’s not strictly needed in some cases. It’s sensible to lube the stabilizers, though, as that at least will lower the pitch of their sound.
Mounting styles can be significant. Whatever the IKKI68 Aurora uses, for example, seems better optimized for dampening than some other systems.
So after you get a “silent switch,” you’ll want to put it into a keyboard that is relatively quiet / dampened. Fortunately, there are several ‘hotswap’ boards like this that you can just drop switches into. In my experience, these boards are the Archon AK87/AK89 [TKL and full-size], the IKKI68 Aurora [this is a 70% board, though], and you can also stuff dampening into a RAKK LAM ANG Pro or even a barebones GMMK TKL, there are mods you can do. There are other platforms as well.
With most of those hotswap keyboards that are relatively quiet, you’ll still want to lube the stabilizers. Most of them use ‘plate-mount’ stabilizers which are easy to remove. You can lube them and put them back, or replace them with lubed, high-grade stabilizers. Some of these keyboards can be retrofitted with PCB-mount stabilizers as well, which can be made fairly quiet.
So, for example, you could buy some U4 Boba switches, put them into an Archon AK89 [which you have lubed / replaced the stabilizers for], and then you have a keyboard noticeably quieter than some MX Brown / Red store-bought board.
I mentioned the IKKI68 Aurora, because it’s almost a TKL [have to use function-layer to get function keys, though], relatively easy to assemble, and yes, you can set it up to be dampened.
Other people here can tell you about soldered kits that can be made quiet. I’m not sure which those are, TBH. I’ve seen a silent Tofu (60%) that’s on Youtube. It’s usually a matter of dampening. Maybe a heavily-dampened KBD8X MKII would be a good match for silent switches, for example. I’ve even seen people put them into an NCR-80, an affordable TKL that I always thought was resonant.
We haven’t even gotten into silenced EC boards, which is a whole other can 'o worms but perfectly legitimate as silenced ‘mechanical’ keyboards.