https://github.com/matrix-org/matrix-react-sdk/pull/7339
    https://github.com/matrix-org/matrix-react-sdk/commit/7fa27f583490a4a9bf0b5469fef6143bc5f5bc06
https://github.com/matrix-org/matrix-react-sdk/pull/7372
    !!! merged, 2022-03-15, /jumptodate slash command at "Settings->Labs"
https://github.com/matrix-org/matrix-react-sdk/pull/7317
    ?! closed ?! right after merging 7339
https://github.com/matrix-org/matrix-js-sdk/pull/2072

https://github.com/matrix-org/matrix-react-sdk/blob/develop/src/settings/Settings.tsx#L349
    "feature_jump_to_date": {
        // We purposely leave out `isFeature: true` so it doesn't show in Labs
        // by default. We will conditionally show it depending on whether we can
        // detect MSC3030 support (see LabUserSettingsTab.tsx).
        // labsGroup: LabGroup.Messaging,
        displayName: _td("Jump to date (adds /jumptodate and jump to date headers)"),
        supportedLevels: LEVELS_FEATURE,
        default: false,
    },
https://github.com/matrix-org/matrix-react-sdk/blob/develop/src/components/views/settings/tabs/user/LabsUserSettingsTab.tsx
    @replaceableComponent("views.settings.tabs.user.LabsUserSettingsTab")
    export default class LabsUserSettingsTab extends React.Component<{}, IState> {
        constructor(props: {}) {
            super(props);

            MatrixClientPeg.get().doesServerSupportUnstableFeature("org.matrix.msc2285").then((showHiddenReadReceipts) => {
                this.setState({ showHiddenReadReceipts });
            });

            MatrixClientPeg.get().doesServerSupportUnstableFeature("org.matrix.msc3030").then((showJumpToDate) => {
                this.setState({ showJumpToDate });
            });

            this.state = {
                showHiddenReadReceipts: false,
                showJumpToDate: false,
            };
        }
        ...
    }
https://github.com/matrix-org/matrix-react-sdk/blob/develop/src/MatrixClientPeg.ts
    import { IStartClientOpts, MatrixClient } from 'matrix-js-sdk/src/client';
    ...
    public get(): MatrixClient {
        return this.matrixClient;
    }
    ...
https://github.com/matrix-org/matrix-js-sdk/blob/develop/src/client.ts
    ...
   /**
     * Query the server to see if it lists support for an unstable feature
     * in the /versions response
     * @param {string} feature the feature name
     * @return {Promise<boolean>} true if the feature is supported
     */
    public async doesServerSupportUnstableFeature(feature: string): Promise<boolean> {
        const response = await this.getVersions();
        if (!response) return false;
        const unstableFeatures = response["unstable_features"];
        return unstableFeatures && !!unstableFeatures[feature];
    }
    ...
   /**
     * Get the API versions supported by the server, along with any
     * unstable APIs it supports
     * @return {Promise<object>} The server /versions response
     */
    public getVersions(): Promise<IServerVersions> {
        if (this.serverVersionsPromise) {
            return this.serverVersionsPromise;
        }

        this.serverVersionsPromise = this.http.request<IServerVersions>(
            undefined, // callback
            Method.Get, "/_matrix/client/versions",
            undefined, // queryParams
            undefined, // data
            {
                prefix: '',
            },
        ).catch((e: Error) => {
            // Need to unset this if it fails, otherwise we'll never retry
            this.serverVersionsPromise = null;
            // but rethrow the exception to anything that was waiting
            throw e;
        });

        return this.serverVersionsPromise;
    }