This slider was removed, some time ago, so that Wwise works to better support best practice use of the .1 LFE channel. If you want to send a sound to LFE, you can do so by using the Wwise Tool - Multi Channel Creator. To create and the import a sound that Wwise recognizes as a specific , .1 LFE sound.
See link to documentation on the tool.
https://www.audiokinetic.com/library/edge/?source=Help&id=using_multi_channel_creator_overview
You can also use the Wwise Tone Generator as an LFE sound, which can be a very effective way of adding additional , thump, for those users fortunate enough to have an LFE channel.
While this may strike you as a limitation, as a best practice, it is preferable to use the LFE as it was intended, as an effect, and not risk the problems that will occur when multi-channel mixes are folded down to systems that do not support .1 channels.
Having unique LFE sounds will allow you more control and predictability of your final mix.
Bear in mind that unless the user has a discrete .1 channel they will not experience the Low Frequency Effect. Listen to your mix in 2.0 Stereo, it may be that the main sound simply misses some low frequency that could be added by judicious use of an eq.