Issue:

On 20 November, 2017, NVIDIA discontinued development of the mental ray and IRay renderers, as outlined in this Nvidia transition statement. More information on how to obtain support for mental ray and IRay can be found at this mental ray plugin FAQ sheet.
However, you would like to know if you can still obtain these renderers for 3ds Max 2018 and Maya 2018 to aid in rendering legacy scene files.
Note: Nvidia has confirmed that mental ray will not be available for 3ds Max or Maya, versions 2019 and higher, and mental ray 2018 installations will not work with 3ds Max 2019 or Maya 2019. Nvidia mental ray 2018 is the last version which works in 3ds Max and Maya.

Solution:

3ds Max 2018 customers who didn't download and install NVIDIA Mental Ray and IRay plugins for 3ds Max before 20 November, 2017, can still download and install them from this Nvidia Forums link.
Maya 2018 customers can still download the NVIDIA Mental Ray and IRay plugins from this NVIDIA Forums link.
For 3ds Max 2018 users who relied on Mental Ray to produce point cloud renderings, please see the article: How to render Point Clouds in 3ds Max 2018.
Note: The versions of Mental Ray available from the links above are for workstation copies only, not network render node licenses. See the NVIDIA Mental Ray Transition FAQ sheet for information on render node licenses.

See Also:

Unable to render with Mental Ray in 3ds Max 2018 and more recent versions
Changes to NVIDIA Mental Ray rendering for 3ds Max 2018 FAQ
Error: 'Missing Dlls' warning appears when opening 3ds Max or Mental Ray scene files

Versions:

2018;