Site Maintenance Mode is a simple plugin that allows you to put your WordPress site into maintenance mode. It will show a maintenance message to all non-logged-in users while allowing logged-in users to access the site normally.
<h3>License</h3>
This plugin is licensed under the GPLv2 or later. For more information, please see <a href="http://www.gnu.org/licenses/gpl-2.0.html" rel="nofollow ugc">http://www.gnu.org/licenses/gpl-2.0.html</a>.

Sim Site Maintenance

Puts the site in maintenance mode for all non-logged-in users.

Lukas Juhas

As a web developer working with WordPress almost every day, very often I came across problem that If I wanted simple maintenance plugin to do some updates on my or client’s website, I came across all these useless and overcomplicated plugins until I decided that I’ll make one myself which will be the one I’ll be confident and happy to use with ease.
<a href="https://plugins.itsluk.as/maintenance-mode/" rel="nofollow ugc">Maintenance Mode</a> is very simple and it’s using WordPress’s wp_die() function which is core function of WordPress, which makes this plugin feel and work as a part of WordPress core. There is settings page under “Settings” in the main wp-admin menu where you can enable maintenance mode or change content using WYSIWYG editor so you can also add pictures, links etc. When activated and logged in as admin, you can see website as usual, rest of the users / visitors can see maintenance mode. You can see the maintenance mode as admin in preview mode using preview link on the settings page or simply open website in private mode or different browser to double check if it’s enabled and working. There is also indicator in admin bar that changes colour to red when maintenance mode is enabled.
Features:
<ul>
<li>Simplicity – Built to be as simple as possible. Easy to use.</li>
<li>Customisable – WYSIWYG available in full glory. You can add images and other media including links and pretty much everything you can do with WYSIWYG. Text / Code tab is available too for custom markup.</li>
<li>Works on mobile – Because of it’s simplicity, maintenance mode works very well on mobile devices.</li>
<li>NO ADS – No ads. Seriously.</li>
<li>NO PAID UPGRADES – No paid upgrades. Seriously.</li>
<li>Updates – Regular updates and maintenance.</li>
<li>Support – Support button available.</li>
<li>Preview – Preview button available.</li>
<li>Compact – It’s developed to be as compact as possible.</li>
<li>Role Control – User Role control is available since 2.0</li>
<li>Optional widgets – Optionally add widgets above and/or below the content</li>
<li>Optional style sheet – Optionally add a custom style sheet</li>
<li>Optional ability to add code snippet – Optionally add a code snippet to the page.</li>
<li>Support for Analytify plugin – If you use the Analytify plugin, you can automatically insert the Google Analytics tracking code.</li>
</ul>
<blockquote>
Important! Users that are using Cache plugins, please read below: 
 When enabling or disabling Maintenance Mode, don’t forget to flush your cache!
</blockquote>
Filters 
 ljmm_site_title – Filter page title while in maintenance mode
<pre><code>ljmm_admin_bar_indicator_enabled - Control visibility of admin bar indicator

limm_css_filename - The filename of the CSS style sheet (as found in the theme's stylesheet directory) - just the filename, for example: `maintenance.min.css`. (Note: you do not need to use this filter for a stylesheet; see FAQs below.)

ljmm_css_url - The url of the css file.
</code></pre>
Actions 
 ljmm_before_mm – Runs at the beginning of core maintenance method
Having trouble? Please read FAQ first, if you need any assistance, you can use support button on the settings page of the Maintenance Mode.
<blockquote>
Development on GitHub 
 The development of Maintenance Mode <a href="https://github.com/lukasjuhas/lj-maintenance-mode" rel="nofollow ugc">takes place on GitHub</a>. Bugs and pull requests are welcomed there.
</blockquote>

Maintenance Mode

WP 5.1.1. Does not work. Activated but no settings pages available. Can't even get to settings via Plugins -> Maintenance Mode -> Settings.

Does not work

... a non-bloated, hassle-free maintenance plugin.

Thank you.

Finally...

Very simple Maintenance Mode & Coming soon page using default Wordpress markup with no ads or paid upgrades.

Sim Site Maintenance vs Maintenance Mode